2.4 electronic mail

(1)

Application Layer 2-1

Chapter 2 part B: outline

2.3 FTP

2.4 electronic mail

SMTP, POP3, IMAP

2.5 DNS

FTP: the file transfer protocol

file transfer

FTP server FTP

user interface

FTP client

local file system

remote file system user

at host

transfer file to/from remote host client/server model

client: side that initiates transfer (either to/from remote) server: remote host

ftp: RFC 959

ftp server: port 21

(2)

FTP: separate control, data connections

FTP client contacts FTP server at port 21, using TCP

client authorized over control connection

client browses remote directory, sends commands over control connection when server receives file transfer command, server opens 2

^nd

TCP data

connection (for file) to client after transferring one file, server closes data connection

FTP client

FTP server

TCP control connection,

server port 21

TCP data connection, server port 20

server opens another TCP data connection to transfer another file

control connection: “ _{out of} band ”

FTP server maintains

“ state ” : current directory, earlier authentication

FTP commands, responses

sample commands:

sent as ASCII text over control channel

USER username PASS password LIST return list of file in current directory

RETR filename retrieves (gets) file

STOR filename stores (puts) file onto remote host

sample return codes

status code and phrase (as in HTTP)

331 Username OK, password required 125 data

connection already open;

transfer starting

425 Can ’ ’ ’ ’ t open

data connection

452 Error writing

file

(3)

Chapter 2 part B: outline

2.3 FTP

2.4 electronic mail

SMTP, POP3, IMAP

2.5 DNS

Electronic mail

Three major components:

user agents mail servers

simple mail transfer protocol: SMTP

User Agent

a.k.a. “ mail reader ”

composing, editing, reading mail messages

e.g., Outlook, Thunderbird, iPhone mail client

outgoing, incoming

messages stored on server

user mailbox outgoing message queue

mail server

SMTP SMTP SMTP

user agent

(4)

Electronic mail: mail servers

mail servers:

mailbox contains incoming messages for user

message queue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send email messages

client: sending mail server

“ server ” : receiving mail server

mail server

SMTP SMTP SMTP

user agent

Electronic Mail: SMTP [RFC 2821]

uses TCP to reliably transfer email message from client to server, port 25

direct transfer: sending server to receiving server

three phases of transfer

handshaking (greeting) transfer of messages closure

command/response interaction (like HTTP, FTP )

commands: ASCII text

response: status code and phrase

messages must be in 7-bit ASCI

(5)

user agent

Scenario: Alice sends message to Bob

1) Alice uses UA to compose message “ to ”

bob@someschool.edu 2) Alice ’ s UA sends message to her mail server; message placed in message queue 3) client side of SMTP opens

TCP connection with Bob ’ s mail server

4) SMTP client sends Alice ’ s message over the TCP connection

5) Bob ’ s mail server places the message in Bob ’ s mailbox 6) Bob invokes his user agent

to read message

mail server

mail server 1

2 3 4

5

6

Alice’s mail server Bob’s mail server

user agent

Sample SMTP interaction

S: 220 hamburger.edu C: HELO crepes.fr

S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr>

S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu>

S: 250 bob@hamburger.edu ... Recipient ok C: DATA

S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup?

C: How about pickles?

C: .

S: 250 Message accepted for delivery C: QUIT

S: 221 hamburger.edu closing connection

(6)

Try SMTP interaction for yourself:

telnet servername 25 see 220 reply from server

enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands

above lets you send email without using email client (reader)

SMTP: final words

SMTP uses persistent connections

SMTP requires message (header & body) to be in 7-bit ASCII

SMTP server uses CRLF.CRLF to

determine end of message

comparison with HTTP:

HTTP: pull SMTP: push both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg

SMTP: multiple objects

sent in multipart msg

(7)

Mail message format

SMTP: protocol for exchanging email msgs RFC 822: standard for text

message format:

header lines, e.g.,

To:

From:

Subject:

different from SMTP MAIL FROM, RCPT TO:

commands!

Body: the “ message ”

ASCII characters only

header

body

blank line

Mail access protocols

SMTP: delivery/storage to receiver ’ s server mail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939]: authorization, download

IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored msgs on server

HTTP: gmail, Hotmail, Yahoo! Mail, etc.

sender’s mail server

SMTP SMTP mail access

protocol

receiver’s mail server

(e.g., POP, IMAP) user

agent

user agent

(8)

POP3 protocol

authorization phase

client commands:

user: declare username pass: password server responses

+OK -ERR

transaction phase, client:

list: list message numbers retr: retrieve message by number

dele: delete quit

C: list S: 1 498 S: 2 912 S: . C: retr 1

S: <message 1 contents>

S: . C: dele 1 C: retr 2

S: <message 1 contents>

S: . C: dele 2 C: quit

S: +OK

POP3 server signing off

S: +OK POP3 server ready C: user bob

S: +OK

C: pass hungry

S: +OK

user successfully logged on

POP3 (more) and IMAP

more about POP3

previous example uses POP3 “ download and delete ” mode

Bob cannot re-read e- mail if he changes client

POP3 “ download-and- keep ” : copies of messages on different clients

POP3 is stateless across sessions

IMAP

keeps all messages in one place: at server

allows user to organize messages in folders keeps user state across sessions:

names of folders and

mappings between

message IDs and folder

name

(9)

Chapter 2 part B: outline

2.3 FTP

2.4 electronic mail

SMTP, POP3, IMAP

2.5 DNS

2.6 P2P applications 2.7 socket programming

with UDP and TCP

DNS: domain name system

people: many identifiers:

SSN, name, passport # Internet hosts, routers:

IP address (32 bit) - used for addressing datagrams

“ name ” , e.g., www.yahoo.com - used by humans Q: how to map between IP

address and name, and vice versa ?

Domain Name System:

distributed database

implemented in hierarchy of many name servers

application-layer protocol: hosts, name servers communicate to resolve names (address/name translation)

note: core Internet function, implemented as application- layer protocol

complexity at network ’ s

“ edge ”

(10)

DNS: services, structure

why not centralize DNS?

single point of failure traffic volume

distant centralized database maintenance

DNS services

hostname to IP address translation

host aliasing

canonical, alias names

mail server aliasing load distribution

replicated Web servers: many IP addresses correspond to one name

A: doesn ’ _{t scale!}

Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.edu DNS servers

umass.edu DNS servers yahoo.com

DNS servers

amazon.com DNS servers

pbs.org DNS servers

DNS: a distributed, hierarchical database

client wants IP for www.amazon.com; 1

^st

approx:

client queries root server to find com DNS server

client queries .com DNS server to get amazon.com DNS server client queries amazon.com DNS server to get IP address for www.amazon.com

… …

Top-Level Domain servers

Authorative DNS servers

(11)

DNS: root name servers

contacted by local name server that can not resolve name root name server:

contacts authoritative name server if name mapping not known gets mapping

returns mapping to local name server

13 root name

“ _servers ” worldwide (labeled from A to M)

a. Verisign, Los Angeles CA (5 other sites) b. USC-ISI Marina del Rey, CA l. ICANN Los Angeles, CA

(41 other sites) e. NASA Mt View, CA f. Internet Software C.

Palo Alto, CA (and 48 other sites)

i. Netnod, Stockholm (37 other sites) k. RIPE London (17 other sites)

m. WIDE Tokyo (5 other sites) c. Cogent, Herndon, VA (5 other sites)

d. U Maryland College Park, MD h. ARL Aberdeen, MD

j. Verisign, Dulles VA (69 other sites )

g. US DoD Columbus, OH (5 other sites)

TLD, authoritative servers

top-level domain (TLD) servers:

responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp Network Solutions maintains servers for .com TLD Educause for .edu TLD

authoritative DNS servers:

organization ’ s own DNS server(s), providing

authoritative hostname to IP mappings for organization ’ s named hosts

can be maintained by organization or service provider

(12)

Local DNS name server

does not strictly belong to hierarchy

each ISP (residential ISP, company, university) has one

also called “ default name server ”

when host makes DNS query, query is sent to its local DNS server

has local cache of recent name-to-address translation pairs (but may be out of date!)

acts as proxy, forwards query into hierarchy

requesting host

cis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS server

dns.poly.edu

1 2

3 4

5

6

authoritative DNS server dns.cs.umass.edu 8 7

TLD DNS server

DNS name

resolution example

host at cis.poly.edu wants IP address for gaia.cs.umass.edu

iterated query:

contacted server replies with name of server to contact

“ I don ’ t know this

name, but ask this

server ”

(13)

5 4 6

3

recursive query:

puts burden of name resolution on

contacted name server

heavy load at upper levels of hierarchy?

requesting host

cis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS server

dns.poly.edu

1 2

7

authoritative DNS server dns.cs.umass.edu 8

DNS name

resolution example

TLD DNS server

DNS: caching, updating records

once (any) name server learns mapping, it caches mapping

cache entries timeout (disappear) after some time (TTL) TLD servers typically cached in local name servers

• thus root name servers not often visited

cached entries may be out-of-date (best effort name-to-address translation!)

if name host changes IP address, may not be known Internet-wide until all TTLs expire

update/notify mechanisms proposed IETF standard

RFC 2136

(14)

DNS protocol, messages

query and reply messages, both with same message format

msg header

identification: 16 bit # for query, reply to query uses same #

flags:

query or reply recursion desired recursion available reply is authoritative

identification flags

# questions

questions (variable # of questions)

# additional RRs

# authority RRs

# answer RRs

answers (variable # of RRs)

authority (variable # of RRs)

additional info (variable # of RRs)

2 bytes 2 bytes

name, type fields for a query RRs in response to query records for authoritative servers additional “ helpful ” info that may be used

identification flags

# questions

questions (variable # of questions)

# additional RRs

# authority RRs

# answer RRs

answers (variable # of RRs)

authority (variable # of RRs)

additional info (variable # of RRs)

DNS protocol, messages

2 bytes 2 bytes

2.4 electronic mail

Chapter 2 part B: outline

2.3 FTP

2.4 electronic mail

SMTP, POP3, IMAP

2.5 DNS

FTP: the file transfer protocol

transfer file to/from remote host client/server model

client: side that initiates transfer (either to/from remote) server: remote host

ftp: RFC 959

ftp server: port 21

FTP: separate control, data connections

FTP client contacts FTP server at port 21, using TCP

client authorized over control connection

client browses remote directory, sends commands over control connection when server receives file transfer command, server opens 2

TCP data

connection (for file) to client after transferring one file, server closes data connection

FTP client

FTP server

server opens another TCP data connection to transfer another file

control connection: “ out of band ”

FTP server maintains

“ state ” : current directory, earlier authentication

FTP commands, responses

sample commands:

sent as ASCII text over control channel

USER username PASS password LIST return list of file in current directory

RETR filename retrieves (gets) file

STOR filename stores (puts) file onto remote host

sample return codes

status code and phrase (as in HTTP)

331 Username OK, password required 125 data

connection already open;

transfer starting

425 Can ’ ’ ’ ’ t open

data connection

452 Error writing

file

Chapter 2 part B: outline

2.3 FTP

2.4 electronic mail

SMTP, POP3, IMAP

2.5 DNS

Electronic mail

Three major components:

user agents mail servers

simple mail transfer protocol: SMTP

User Agent

a.k.a. “ mail reader ”

composing, editing, reading mail messages

e.g., Outlook, Thunderbird, iPhone mail client

outgoing, incoming

messages stored on server

SMTP SMTP SMTP

Electronic mail: mail servers

mail servers:

mailbox contains incoming messages for user

message queue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send email messages

client: sending mail server

“ server ” : receiving mail server

SMTP SMTP SMTP

Electronic Mail: SMTP [RFC 2821]

uses TCP to reliably transfer email message from client to server, port 25

direct transfer: sending server to receiving server

three phases of transfer

handshaking (greeting) transfer of messages closure

command/response interaction (like HTTP, FTP )

commands: ASCII text

response: status code and phrase

messages must be in 7-bit ASCI

Scenario: Alice sends message to Bob

1) Alice uses UA to compose message “ to ”

bob@someschool.edu 2) Alice ’ s UA sends message to her mail server; message placed in message queue 3) client side of SMTP opens

TCP connection with Bob ’ s mail server

4) SMTP client sends Alice ’ s message over the TCP connection

5) Bob ’ s mail server places the message in Bob ’ s mailbox 6) Bob invokes his user agent

to read message

Sample SMTP interaction

S: 220 hamburger.edu C: HELO crepes.fr

S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr>

control connection: “ _{out of} band ”